Communications between radio communication devices such as portable radios, mobiles, and fixed equipment are readily susceptible to interception by eavesdroppers. If security of communications is important to radio communication equipment users, it can be achieved with a system such as the Motorola Digital Voice Protection System. This is a system that converts an analog communication signal into a digital bit stream by use of a continuously-variable-slope delta modulator ("CVSD") which is a bit synchronous analog to digital conversion technique. The digital bit stream is then scrambled (encrypted) by a system to which only the sender and authorized receivers are given "keys" which allow the signal to be decrypted. When such a system is in use, the eavesdropper may detect the signals, but he receives no more than a pseudorandom signal resembling noise, since he lacks the means to decrypt the signals.
The user of a two-way radio having a digital encryption system such as the Motorola Digital Voice Protection System needs to be able to receive both "clear" (non-encrypted) communication messages, and encrypted binary form messages since a radio user wants to be able to receive both clear or encrypted messages automatically. The detection of the presence of digital signals can be carried out by a system such as that of U.S. Pat. No. 3,995,225. The patent is incorporated by reference as if set forth fully within.
If the receiver circuitry has detected the presence of an encrypted signal, it is next necessary to submit the signal to an unscrambling process. This requires that the user have available a decrypting system containing the same encryption algorithm (encryption algorithm refers to an algorithm which is used to decrypt or encrypt encrypted messages) and the specific key with which the message was encrypted. If the radio has such a system, and the message was applied to the system the encrypted message will be decrypted and will be heard as ordinary voice and/or data at the communication device receiving the message. If the user did not have the proper encryption algorithm and specific key the result would be to produce a signal with noise-like properties, which would be unintelligible. A decrypted signal can be distinguished from a signal that has not been decrypted by a circuit such as the one in U.S. Pat. No. 4,197,502. This patent is incorporated by reference as if set forth fully herein. The circuit of that patent provides a signal which can be used to mute a receiver if a digital signal has not been decoded.
In a typical encrypted communication system, groups of portable radios or mobile units communicate with each other using the same encryption algorithms, having the same specific algorithm keys. Frequently it is desirable for supervisory radios to be able to listen to different groups of users with each group having their own different encryption algorithms. A supervisor, for example, might wish to receive and talk back to scrambled messages coming in on one specific channel from different users or user groups. For example, a supervisor may wish to receive transmissions from a group of users using one type of encryption algorithm (i.e. DVP-XL.TM., trademark of Motorola, Inc.) and also be able to receive messages on the same channel from another group of users using another encryption algorithm (i.e. DES-XL.TM., trademark of Motorola, Inc.). Presently, the only way a user could receive the messages from the two different groups is to manually switch the encryption algorithm on the radio, or else the radio would miss one of the messages assuming the radio had one specific algorithm selected. Each of the different encryption algorithms on the radio can also carry a plurality of encryption keys, the radio can automatically choose among different keys of a particular encryption algorithm by using the technique described in U.S. Pat. No. 4,440,976. The patent is incorporated by reference as if set forth fully herein. This patent describes how to automatically pick from a group of keys belonging to one specific encryption algorithm (i.e. using DES-XL.TM. encryption and picking from a plurality of different keys).
A need however exists in the art for a method and means for automatically selecting between different encryption algorithms (e.g. between DES-XL.TM. and DVP-XL.TM., etc.). For example, different encryption algorithms such as DVP-XL.TM., and DES-XL.TM. could be utilized in one radio and the ability of automatically selecting the correct algorithm to use in order to decrypt an incoming signal would be very useful especially for supervisory radios which need to communicate with groups of radios utilizing different encryption algorithms.